1. Field of the Invention
This invention relates generally to electrical overvoltage protective circuitry and more particularly, it relates to a sidactor protector for protecting telecommunication equipment against high voltage surges which includes an improved shunt protection arrangement for the sidactor protector.
2. Description of Prior Art
A state-of-the-art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed British patent application No. 2,167,915 and the following U.S. Pat. Nos.:
______________________________________ 2,413,887 4,635,091 3,023,289 4,717,902 3,123,696 4,858,059 3,710,297 4,910,489 4,047,143 5,029,302 4,233,641 5,089,929 4,371,911 ______________________________________
In U.S. Pat. No. 4,910,489 to H. Neuwirth et al. issued on Mar. 20, 1990, there is disclosed a fail-safe secondary fuse device 20 for assuring the grounding of a conductive gas tube used in modular protection devices for individual subscriber circuit pairs. The device comprises a length of spring music wire 21 having a central loop adapted to surround the central electrode 16 of the gas tube and rectilinear legs 23 and 24 terminating in ends 25 and 26. The end electrodes 14 and 15 of the gas tube are insulated from direct electrical communication with the respective ends 25 and 26 of the device by a fusible sleeve 27 made of insulative material. Upon the occurrence of a continued current overload, the heat emanating from the gas tube will serve to fuse the sleeve 27 thereby permitting the ends 25 and 26 to short the end electrodes 14 and 15 to the central electrode 16 which is grounded.
In U.S. Pat. No. 4,717,902 to K. S. James issued on Jan. 5, 1988, there is disclosed an excess voltage arrestor which is fitted with a protective temperature responsive device formed of a wire 1 of spring temper. The wire has a single loop 2 and is coated with a polyurethane varnish 3. The loop 2 is disposed over the central pin 4 of the voltage arrestor while the spring arms thereof are retained in a stressed condition by resting on the two outer pins 5 of the voltage arrestor. At an elevated temperature, the coating of the polyurethane varnish decomposes so as to allow the bared wire 1 to make electrical contact between the pins 4 and 5.
There is disclosed in U.S. Pat. No. 4,858,059 to M. Okura issued on Aug. 15, 1989, a short-circuit device of a gas-filled triple-pole discharge-tube type arrestor for telephone line use which includes a coil-like resilient short-circuit lead 7 being spot welded at its middle portion to an earth ground lead 5 and whose end portions thereof are disposed in a pushing contact made with the leads 3,3 in an entwined condition. The pushing contact portions of the leads 3,3 in contact with the lead 7 are coated with a low temperature meltable insulator 8,8.
There is disclosed in British patent application No. 2,167,915 to J. P. Phillips et al. published on Jun. 4, 1986, a circuit protection arrangement which includes a switch contact 4 in the form of a shaped resilient wire. The middle portion of the wire is tensioned so that it is biased towards the central earthed pin 2 of a gas discharged tube 1 but is constrained into contact with a pin 3 and post 7 by a fusible joint at point P1. When the temperature of the gas discharged tube 1 rises sufficiently so as to melt the fusible alloy at the point P1, the switch contact 4 is released from its constrained position so as to short the output terminals E1 or E2 to ground (FIG. 2a).
The remaining patents listed above but not specifically discussed are believed to be of only general interest and show the state of the art in overvoltage protection devices for protecting expensive equipment which incorporates various fail-safe mechanisms.
With the advent of electronic circuits for use in telecommunication equipment, there has arisen a need of providing new types of overvoltage protection means for the electronic circuits since they cannot tolerate the overvoltages which were permissible heretofore. As is generally known, the purpose of the overvoltage protection circuits is to protect the expensive telecommunication equipment connected to the output side of a terminal circuit against high voltage surges caused, for instance, by lightning strikes on the subscriber line.
A known conventional overprotection device in widespread use is the so-called three-element gas tube having a pair of spaced apart end electrodes and a central electrode. The end electrodes are typically connected to a pair of output lines coupled to the telephone equipment which is desired to be protected against excessive voltage, and the central electrode is connected to earth ground. Upon the occurrence of a voltage between the end electrodes or between either of the end electrodes and the central electrode above a predetermined potential, the gas tube becomes electrically conductive so as to shunt the overvoltage to ground thereby protecting the telecommunication equipment from the excessive voltage. However, in the event of a sustained over-voltage, over-current condition, that is, power crossing, the gas tube remains conductive and becomes overheated causing a fire hazard. Accordingly, it is important that the gas tube should fail in a safe manner so as to avoid leaving the telephone equipment unprotected.
Therefore, it has also been developed heretofore in the prior art various types of fail-safe arrangements for use in association with gas tubes and other kinds of overvoltage protection devices such as air-gap arrestors and the like. One form of fail-safe arrangement includes a temperature-responsive device comprising a resilient electrically conductive member which is normally maintained in a stressed condition by means of a heat softenable material. However, when the heat softenable material is used to normally hold the stressed resilient electrically conductive member out of engagement with a cooperative contact and is subsequently melted so as to permit a short-circuit engagement between the conductive member and the contact, there always exists the possibility that the stress applied to the conductive member will not be adequate enough in order to cause the conductive member to shunt the overvoltage to ground within a predetermined time interval.
Further, in a still later development the known three-element gas tubes have been generally replaced by solid-state voltage suppressors referred to as "sidactors" which are of a smaller dimension than the gas tubes. The sidactors are formed with legs so that a plurality of them can be mounted in corresponding holes formed in a printed circuit board. As a result, the telephone connector blocks incorporating the printed circuit boards can be fabricated with an even higher circuit density. Therefore, it has been necessary to provide an improved fail-safe shunt protection arrangement for assuring that sufficient pressure is applied so as to ground the sidactor at elevated temperatures without significantly increasing the amount of space needed.
The present invention represents an improvement over the aforementioned '489, '902, and '059 U.S. Pat. Nos. and the '915 British patent application. The invention utilizes a torsional type spring whose spring arms are initially formed so as to have an obtuse angle therebetween. Further, during assembly, the center of the single loop portion of the torsional spring is disposed off-centered in relation to the centers of the end terminal pins of the sidactor in order to insure that sufficient and constant pressure is applied between the spring arms and the end terminal pins.